Detergent compositions usually contain, in addition to a detergent active material, a detergency builder whose role, inter alia, is to remove hardness ions from the wash liquor which would otherwise reduce the efficiency of the detergent active material. Water-soluble phosphate materials have been extensively used as detergency builders. However for a number of reasons, including eutrophication allegedly caused by phosphates and cost, there has been a desire to use alkali metal carbonates especially sodium carbonate instead. Alkali metal carbonate detergency builders suffer however from a number of disadvantages. Firstly, the reaction between the alkali metal carbonate and calcium ions which are present in hard water results in the formation of water-insoluble calcium carbonate which, depending on the conditions, may be in such a form as to become deposited on the washed fabrics. Secondly, the reaction between the alkali metal carbonate and the calcium ions of the water is slow, especially at low temperatures and is readily inhibited by materials which act as calcium carbonate precipitate growth inhibitors, referred to herein as poisons. The result of this is that the concentration of calcium ions in the wash liquor is not reduced as far or as fast as desired, so that some free calcium ions are still available to reduce the efficiency of the detergent active material.
As a possible solution to this problem it has been proposed to include in the detergent composition, a water-insoluble material which would act as a seed crystal for the precipitated calcium carbonate and would adsorb the poisons from the wash liquor. Among other materials, finely divided calcite has been proposed as such a material - see British Patent Specification GB 1 437 950 (UNILEVER).
However, the inclusion of calcite in detergent compositions is hampered by its physical form. One might consider putting small particle size calcite in a slurry together with other ingredients for spray-drying, but we have found that where alkali metal silicates are included this process leads to a loss of calcite seed activity as a result of poor dispersibility. Calcite having a large surface area is preferred for maximum seed activity, but generally such material has a relatively small particle size, is dusty and is therefore difficult to handle. One alternative is to handle the calcite in a slurry, without drying to a powder, but this could also involve high storage and transport costs. It is therefore necessary to granulate the calcite, for example by conventional techniques of pan granulation or spray-drying, and to keep any silicate away from the calcite. The term "granulation" is used herein to mean any process of agglomerating fine particles into granules of a suitable size for incorporation into, or use directly as, detergent compositions.
Granulation of the calcite with a suitable binding agent has been proposed, for example in British Patent Specification GB 1 515 273 (UNILEVER). However, in order to be effective in its intended role in the wash liquor, it is necessary for the calcite to disperse rapidly when the product is added to water. Binding agents have generally been found to seriously reduce the dispersibility of the calcite.
Attempts to granulate calcite with materials known to be good dispersing agents, for example some nonionic detergent active materials, have also not been successful. The resulting granules may not have the necessary mechanical strength to solve the handling problems of the calcite. Attempts to discover a material which will act both as an adequate binding agent and a dispersant have not so far been successful.
The problem is further complicated by the fact that some binding agents and dispersing agents proposed in the prior art are themselves poisons and will therefore reduce the seed activity of the calcite, thereby further adding to the problems which the calcite is intended to solve.